compression

[plehman]

The new to me R90/6 1976. Reliable mechanic did a simple compression test. Bad news. 60 in one, 100 in the other. The valves were way! off, so we adjust them warm (I know, will do it cold), but this seems very bad. Shouldn't they be even and at least 120? What contributes to low compression? Only 35,000 miles.

The push rod housing on the bad cylinder is flipped around. That can't be good but shouldn't really contribute. The guy who I bought it from said he did the valves and replaced the pistons. He is a mechanic but I'm not very confident in his abilities.

thanks

[crazydrummerdude]

If the valves were off, the compression test values are worthless. Adjust them, and check it again. I'll bet you'll be pleasantly surprised.

..and the second paragraph of yours doesn't make sense to me. Got a pic?

[plehman]

I'll take one tonight. It's kinda funny. There's an asymmetrical fitting up against the crank case. He has the fat end down - should be the other way around. I guess I have to pull the cylinder head and the barrel to turn the push rod tube housing.

And I sure hope you're right about the valve adjust. Thanks!

[RandyB]

Make sure the throttle is wide open on both tests. I nearly gave myself a heart attack on my GS when I didn't notice it closed as I was testing one cylinder. Wildy disparate readings.

I now remove the carbs to test.

[carockwell]

I have done, no kidding, over 4000 compression tests. If I sold a new engine for every one that flunked.....the moral of the story is that compression tests are very unreliable, as are leak down tests. A compression test needs to be done with a warm engine, the throttles AND slides open, and maybe the valves adjusted correctly. A compression test is quite valuable when one cyclinder is hitting only 30 psi or something as it is obviously not working right. In instances when an engine is running pretty good, but the compression is low, like your case, it is food for thought, nothing more. Wait a while and do another test and see if you get the same result. Meanwhile, get the bike tuned and sorted out as best you can before letting the compression test tell you to tear anything down.

[boxermaf]

I agree with the others - adjust the valves properly (cold). THEN warm the engine up fully before doing the compression test. You will definitely get higher numbers if you take the carbs off to do the test properly, but even with them on and held absolutely wide open, you should see at least 120, ideally.

[plehman]

Here's the other red herring. The upside down push rod tube. Maybe it's just bushing out of position. How do you get at that thing?

[RoboRider]

If you have 60/100 (and it is real), then I'd suspect you'd have an engine that runs poorly. How does she run?

[amiles]

Are you sure that 35,000 mi is correct on a 32 yo machine? Might be a bit more.

[mymindsok]

Are you sure that 35,000 mi is correct on a 32 yo machine? Might be a bit more.

Yeah.... Like 235,000 mi! :laugh

[plehman]

It doesn't run so well right now. That's why we checked the compression.
My mentor, Duncan of Duncan's Beemers thinks 35,000 might be accurate because of a few little tell tales.

The center stand tabs look really square. A higher milage bike wears that point.
The little rails on the seat are too clean. Stuff like that.

It seems idleness is worst problem.

[boxermaf]

I have heard of the small braze/weld that hold that flared part onto the pushrod tube failing on one or two bikes, causing it to slip upward and out of place. But, it that isn't the case with this one, from that one photo it does look like it is not put on correctly and/or the rubber seal is on upside down (there is a top and a bottom to those rubber seals).

I am sure that Duncan could quickly figure it out.

[crazydrummerdude]

Here's the other red herring. The upside down push rod tube. Maybe it's just bushing out of position. How do you get at that thing?

If it's just the seal, you can do what I did on my R75/6:

http://img.photobucket.com/albums/v469/aretnap69/bmw/20080417012.jpg